I'm sure there's logic built into AOS that says if the value is blank than use a default value (I assume 1576 based on the link provided). I can't find anything in the reference guides as conclusive proof, but I think you would find this to be true if you removed the current value and performed a packet capture where the AP is connected.
--- Edit ---
Just found the following command that should show you the SAP value. You could try deleting the current value and then issue this command:
show ap debug received-config ap-name XXXXXX
#7240